1. Field of the Invention
The present invention relates generally to a DC/DC converter.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs.
Typically, a vehicle propelled by an electric motor can use batteries or fuel cells to generate the necessary current. Fuel cells generate electrical power through an electrochemical reaction of a fuel and oxidant, such as hydrogen and oxygen. Water is the product of the electrochemical reaction in a fuel cell utilizing hydrogen and oxygen, a product that is easily disposed. See generally, U.S. Pat. No. 5,991,670 to Mufford.
The desirability of using electric motors to propel a vehicle is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or drive-ability. Nevertheless, new ways must be developed to optimize these potential benefits.
One such area of electric vehicle (EV) development is converting direct current (DC) from generating devices such as fuel cells and high voltage (HV) batteries to an appropriate current for driving a load. Ideally, the current generators (such as HV batteries or fuel cells) and loads (such as vehicle 12V powered accessories) would all be at the same voltage level. Unfortunately, this is not presently the case. For example, electric vehicles typically employ a dual-voltage power system, including a conventional 12V voltage system to power conventional 12V loads such as lights, sensors and controllers and a high voltage bus (for example 300V) to power the traction inverter and motor. It is particularly advantageous if energy can to be transferred bi-directionally between the two voltage systems.
Therefore, a successful implementation of electric traction motor propelled vehicles may require an effective bidirectional DC/DC converter. A bi-directional converter may allow the high voltage bus to be used as a current load during start-up or as a current generator, for example during a breaking or slowing of the vehicle. Similarly, a bidirectional converter may allow the 12V battery to be used as a current generator or as a load while charging. Unidirectional and bi-directional DC/DC converters are known. See generally, U.S. Pat. No. 5,745,351 to Taurand and U.S. Pat. No. 3,986,097 to Woods.
In a bidirectional DC/DC converter, the primary side of the transformer can be current-fed and the secondary side can be voltage-fed. The primary side normally experiences a high voltage overshoot when turning a pair of switches off, such as when turning off a pair of switching diodes diagonally opposed across from one another in a bridge circuit. This voltage spike needs to be clamped to avoid the voltage overshoot passing through the switching devices. A passive clamp converter employs a diode and a capacitor to absorb excessive energy from the voltage overshoot and a resistor to dissipate the absorbed energy. Unfortunately, the use of a simple prior art passive-clamped snubber circuit results in severe limitation in a low voltage (e.g., 12V), high current (e.g., hundreds of amperes) application due to significant power loss, although it is a simple approach widely used to resolve the voltage spike issue.
An active clamp in the prior art replaces the resistor in the passive clamp circuit with a switch to pump back the energy to the source when the capacitor is not absorbing energy. This recycles the dissipated energy and improves efficiency, but this technology is expensive to implement.
In the prior art, bidirectional flyback converters are known to be best suited for low power applications. DC/DC converters for use in automobiles must be able to withstand the extreme environmental conditions and higher power requirements experienced by many vehicles. Therefore, there is a desire and a need for an efficient and cost effective high power bidirectional DC/DC converter.